Various articles incorporate ceramic-metal joints and/or seals. This invention relates to ceramic-metal joints and hermetic seals using niobium coatings. It also relates to methods of forming such joints and seals. It specifically relates to joints and seals formed between ceramic and titanium.
It is ordinary practice in joining ceramic to metal to first metallize the ceramic. For example, when joining alumina to titanium, a coating of titanium may be sputtered onto the alumina workpiece following which the titanium workpiece is brazed to the coated alumina with a suitable brazing material such as gold. In the case of sputtered titanium on alumina, the coefficient of expansion for the titanium is 94.times.10.sup.-7 cm/cm/.degree.C. as compared to 69-71.times.10.sup.-7 cm/cm/.degree.C. for the alumina. This differential is a source of stress and cracking, particularly as coating thickness increases.
It is a purpose of this invention to use niobium as the ceramic coating whereby the stresses are greatly reduced and the risk of crack formations is minimized. This results in higher quality joints and, as pointed out below, in higher quality hermetic seals.
It is also difficult to provide a sound joint between ceramic and titanium because the titanium tends to migrate into the braze material used for forming the joint between the ceramic and the titanium. Typically, the preferred braze material for joining or sealing ceramic and titanium components has been gold. The migration of titanium into the braze material eg., gold, causes changes in the properties of the brazed joint or seal. For example, in the case of gold, it causes embrittlement and high stresses in the gold. The result is that such seals and joints develop micro and macro cracks. In the case of seals, this is a threat to hermeticity.
One approach to solving these difficulties has been found in the prior art use of platinum components which serve as a barrier to the titanium migration. However, platinum components are initially expensive and their assembly is time consuming.